The present invention relates to a piezo-electric bimorph type transducer for shifting a magnetic head in order to provide tracking correction, which is suitable for use with magnetic recording and reproduction apparatus, in particular, for use with a rotary head helical-scan-type video tape recorder.
As an apparatus for recording and reproducing signals having a broad frequency band, rotary-head-type magnetic record and reproduction apparatus, the so-called video tape recorder (hereinafter referred to as the VTR), is known.
In the VTR, a magnetic head, which works as a converter for recording and reproduction, is disposed around a rotary disc or a rotating drum, constituting a rotating head.
The thus constructed rotating head is rotated, so that signals are recorded in discontinuous signal-recording tracks formed on a magnetic tape, which tracks are inclined at a predetermined angle with respect to the running direction of the magnetic tape. The signals recorded are also reproduced from the tracks. The VTR is designed so as to be capable of recording and reproducing signals having a broad frequency band, such as television signals, without increasing the moving speed of the magnetic tape significantly. Conventionally, a VTR of that type in which the signal-recording tracks are formed at a comparatively great angle (for example, nearly at a right angle), with respect to the moving direction of the magnetic tape, has been most widely used.
From the view point of simplicity in handling the magnetic tape, it is preferable that the magnetic tape not be very wide. Therefore, in the above-mentioned conventional type VTR, the length of the discontinuous signal-recording tracks is limited, so that it is extremely difficult to record television signals for one field continuously in each single signal-recording track. Therefore, usually signals for one field are separately recorded in a plurality of signal-recording tracks by the so-called segment-type recording method. Furthermore, the magnetic tape has to be brought into close contact with the rotating head, with the magnetic tape being curved in the shape of an arc in the direction of the width of the tape. However, when the winding angle of the magnetic tape around the rotating head increases beyond a certain angle, it becomes difficult to move the magnitic tape accurately and stably and to load the magnetic tape on to the rotating head properly. In order to overcome those difficulties, conventionally a plurality of rotating heads (for example, four rotating heads) are disposed and successively employed for continuous recording or for continuous reproduction.
Therefore, in this type VTR, switching of the rotating heads during recording or reproducing the signals for one field is unavoidable. Such switching of the rotating heads is apt to bring about the so-called banding, causing significant deterioration of the image quality if recording and reproduction are repeated. Furthermore, in the VTR, it is extremely difficult to perform tricky reproductions including slow motion, still, reverse and quick access by changing the the running conditions of the magnetic tape. Therefore, an additional memory device has to be employed for the above-mentioned purposes. Furthermore, the recorded image cannot be monitored during quick access and reverse access.
In order to improve on the above-mentioned shortcomings of the conventional VTR, in which signal-recording tracks are formed at a comparatively small angle (for example, 2 to 4 degrees) with respect to the running direction of the magnetic tape. The VTR of this type has given solutions to most of the problems of the conventional VTR.
In the helical-scan-type VTR, a magnetic head is mounted on a rotating drum having a comparatively large diameter (for instance, a drum having a diameter of about 135 mm). Scanning is done with a magnetic tape slantingly wound around the peripheral surface of the rotating drum. In the VTR, the winding path of the magnetic tape around the rotating drum can be as large as about 360 degrees around the drum. Therefore, it is possible to perform recording and reproduction substantially continuously by use of one magnetic head. Further, each continuous signal-recording track is comparatively long, for example, as long as about 420 mm. Therefore, television signals for one field can be stored in each continuous, single signal-recording track, so that a non-segment type VTR can be constructed by the above-mentioned system. The non-segment type VTR is advantageous on the following points: It is free from the so-called banding. The tricky reproductions can be done without any difficulty. The recorded images can be monitored during quick access and reverse access. Therefore, the VTR of this type will be most widely used in the near future.
As mentioned above, when signals are recorded in the helical-scan-type VTR (hereinafter referred to as the H-VTR), 400 mm or longer signal-recording tracks are formed side by side in the magnetic tape at an inclination of 2 to 4 degrees with respect to the running direction of the magnetic tape. When reproducing the recorded signals, the signal-recording tracks have to be traced accurately. Therefore, extremely high mechanical accuracy is required for the scanning system, including the rotating drum, a magnetic tape driving system and an operation control system. Such mechanical accuracy has to be maintained under any changes in ambient conditions, including temperature and humidity. Therefore, in order to produce practically useful H-VTR's, various sophisticated techniques are required.
Even if the magnetic tape can be driven with sufficiently high accuracy and the magnetic tape can be scanned perfectly by a rotating head by use of high mechanical techniques, it will still be extremely difficult to locate the signal-recording track patterns formed on the magnetic tape with accuracy comparable to the accuracy attained in the mechanical parts of the VTR, including the magnetic tape driving and scanning means.
More specifically, due to the characteristics of the materials which form a tape base of the magnetic tape, it is almost impossible to eliminate any and all changes in size of the magnetic tape, since the size will be changed, for instance, in accordance with change in the ambient temperature or humidity, and changes in tension applied to the magnetic tape or deterioration of the tape with time.
Therefore, it will be extremely difficult to perform the proper tracking of the magnetic tape by a magnetic head for reproducing signals recorded in the magnetic tape, if a long period of time has passed after the recording or if the recording was done under conditions different from those of the reproduction.
Furthermore, it will be impossible to produce a plurality of H-VTR's exactly of the same accuracy and to maintain them at the same accuracy over a long period of time while in use, even if the best techniques are employed. Moreover, it could happen that one magnetic tape is used on a plurality of H-VTR's which are slightly different in mechanical accuracy. In this case, it will be difficult to perform the proper tracking of the magnetic tape in each H-VTR by the magnetic head.
In order to improve on the above-mentioned shortcomings, methods of employing an automatic tracking apparatus for the H-VTR have been proposed in U.S. Pat. Nos. 4,093,885, 4,106,065, 4,151,570 and 4,165,523.
These automatic tracking apparatuses cause a rotating head to trace the signal-recording tracks consistently accurately by automatically correcting its tracking during reproduction of recorded signals.
An automatic tracking apparatus of the above-mentioned type comprises a detection and control means for detecting whether or not the magnetic head is accurately tracing the proper signal-recording track and for producing control signals in accordance with that detection, and a shifting means for shifting the magnetic head, in accordance with the control signals, in the direction normal to the length of the recording tracks.
Referring to FIG. 1 to FIG. 3, an example of such a magnetic head shifting apparatus for use in such an H-VTR will not be explained.
In FIG. 1, reference numeral 1 designates a rotating drum; reference numeral 2, a guide drum; reference numeral 3, a magnetic tape (hereinafter referred to as the tape); reference numeral 4, a magnetic head; reference numeral 5, a guide post on the tape-inlet side; and reference numeral 6, a guide post on the tape-outlet side. Symbol A represents an original point where the magnetic head 4 starts to scan the tape 3.
The tape 3 is wound around the guide drum 2 and the rotating drum 1 so as to cover the two drums 1 and 2 in a spiral manner, with an inner magnetic layer of the tape 3 being in contact with the outer peripheral surfaces of the drums 1 and 2, as shown in FIG. 1. The tape 3 is maintained at a predetermined position by the two guide posts 5 and 6 so as to be capable of performing the predetermined running.
The magnetic head 4 is attached to a lower surface of the rotating drum 1 and is projected slightly from between the rotating drum 1 and the guide drum 2 to the outer peripheral surfaces of the two drums 1 and 2.
When the rotating drum 1 is rotated in the direction of the arrow, the magnetic head 4 scans the magnetic layer of the tape 3 along a predetermined track pattern. In normal operation, the tape 3 is also running in the direction of the arrow, whereby the track patterns, which are formed side by side, are successively scanned.
The magnetic head 4 is maintained by a transducer at such a position as to be capable of deviating in the direction substantially parallel to the rotating shaft of the rotating drum 1. By controlling the deviation, automatic tracking is carried out. That mechanism is shown in FIG. 2.
FIG. 2 is a sectional side view of part of the rotating drum 1 shown in FIG. 1. In FIG. 2, reference numeral 7 represents a piezo-electric bimorph element which constitutes the transducer; reference numeral 8, an attachment member; reference numeral 9, lead wires from the piezo-electric bimorph element 7; and reference numeral 10, signal lead wires from the magnetic head 4. The details of the bimorph element 7 are shown in FIG. 3. In the figure, reference numerals 7a and 7b are rectangular piezo-electric thin plates, made of ceramic plates with piezo-electric characteristics, for instance, plates made of barium titanate; and reference numeral 7c, 7d and 7e, electrode plates made of a conductive film.
When a voltage is applied to the lead wires 9, one of the piezo-electric thin plates 7a and 7b extends and the other shrinks, so that the bimorph element 7 is bent. The bending direction and the bending distance of the bimorph element 7 can be controlled as desired by the voltage applied to the lead wires 9 and by the polarity of the applied voltage. The bimorph element 7 constitutes a cantilever with a fixed end supported by the attachment member 8 and with a movable end to which the magnetic head 4 is attached. Thus, the magnetic head 4 can be moved in the directions of the arrow as shown in FIG. 2 and FIG. 3 in accordance with the polarity and magnitude of voltage applied to the lead wires 9, whereby the magnetic head 4 can be shifted to the desired position, so that the tracking of the track patterns formed on the tape 3 can be changed as desired.
When the magnetic head 4 deviates from the track pattern, that deviation is detected by some means to produce a control signal indicating the deviation, and the control signal is applied to the lead wires 9, so that the deviation of the tracking is corrected.
As can be seen from the above, in the magnetic head shifting apparatus for tracking correction for use in the H-VTR, a cantilever type transducer comprising a piezo-electric bimorph element is generally used, taking into consideration centrifugal acceleration generated during recording and reproduction operations and the effect of the centrifugal acceleration on the magnetic head. One of the requirements for such a transducer to be employed for the above-mentioned purpose is that it be excellent in transient response.
In such a transducer, some time lag due to displacement of the magnetic head and the bimorph is unavoidable. However, it is preferable that adverse vibrations of the magnetic head and the bimorph, generated by the displacement thereof, become zero within a certain period of time.
In order to reduce the adverse vibrations generated by the displacement of the magnetic head and the bimorph, generally a signal for attenuating such vibrations (hereinafter referred to as the attenuation signal), produced by an electric circuit, is caused to overlap with a control signal, as in the method proposed in U.S. Pat. No. 4,080,636. However, in a conventional bimorph, its bending effected by the control signal is not in static and dynamic conformity which will be described in detail later. Therefore, when the vibrations of a certain portion of the bimorph, for example, of a portion near the attachment portion of the magnetic head, are nulified by the attenuation signal, the vibrations of other portions of the bimorph do not become zero at the same time.
Furthermore, the rigidity of the piezo-electric bimorph which constitutes a transducer is limited and, as a matter of fact, the rigidity cannot be increased beyond a certain limit, in order to maintain practical sensitivity. Moreover, the bimorph and the magnetic head respectively have their particular weight. Therefore, in a cantilever type transducer of the above-mentioned type, the level of the control signal and the displacement of the magnetic head do not correspond accurately when the speed of the change in the level of the control signal applied to the transducer, that is, the frequency of the control signal, increases beyond a certain value. Therefore, the conventional piezo-electric bimorph type transducer has the shortcoming that the accurate correction of tracking is extremely difficult.
The above-mentioned phenomenon will now be explained in more detail. When a control signal is applied to the bimorph element, the bimorph element is bent in accordance with the voltage level of the control signal, changing the position of the movable end. However, as mentioned above, the bimorph element and the magnetic head have each their specific mass and inertia. Therefore, when the frequency of the control signal increases beyond a certain frequency, the movable end of the cantilever type bimorph element is moved with complicated mortion, which is difficult from the waveform of the control signal applied to the bimorph element.
In other words, when the change in the voltage of the control signal with time are made moderately, substantially eliminating the effect of the above-mentioned inertia (which state is referred to as the static displacement), the bent state of the entire bimorph element does not conform to the bent state of the entire bimorph element when the changes in voltage are made speedy and the above-mentioned inertia becomes conspicuous (which state is referred to as the dynamic displacement).
Hereinafter, the state of this nonconformity is referred to as the static and dynamic nonconformity state, while the state in which the static displacement and the dynamic displacement are in conformity with each other is referred to as static and dynamic conformity.
Referring to FIG. 4 and FIG. 5, there are shown the bent states of the cantilever type bimorph element. FIG. 4 shows the static and dynamic conformity state, while FIG. 5 shows the static and dynamic nonconformity state. In each of the figures, the broken lines indicate the bending of the bimorph element.
As can be seen from FIG. 5, in the static and dynamic nonconformity state, the displacement of the movable end of the bimorph element does not necessarily indicate its bent state. Therefore, the magnetic head cannot be moved in exact synchronization with the waveform of the control signal. As a result, correction of the tracking becomes difficult.
Moreover, in the static and dynamic nonconformity state, excess bending of the bimorph element takes place at its movable end, and the magnetic head is directed in a direction other than the predetermined direction. As a result, the contact end of the magnetic head does not come into close contact with the magnetic tape in the operation gap, degrading the recording and reproduction characteristics, causing abnormal vibrations in the magnetic head, and damaging the bimorph element.